1. Technical Field
The present invention relates to a method of fabricating a photo mask, and more particularly, to a method of fabricating a chrome-less phase shift mask having no light-shielding film formed of chrome.
2. Discussion of the Related Art
Among processes for semiconductor fabrication, a photolithographic process of forming a fine circuit on a semiconductor substrate is critical. The productivity and quality of a semiconductor device depend on the process capability of the photolithographic process. In the photolithographic process, a photo mask is used to form a given circuit pattern on a semiconductor substrate. A chrome mask is an example of a photo mask. The chrome mask includes a mask body and a chrome light-shielding film. The mask body is formed of a transparent material, such as quartz. The chrome light-shielding film has a circuit pattern to be transferred. That is, the chrome mask has the chrome light-shielding film that in turn has the circuit pattern to be transferred, which is formed on a surface of the mask body. The chrome light-shielding film shields light irradiated in an exposure process so that the circuit pattern is formed in photoresist coated on the semiconductor substrate. However, the circuit pattern should be scaled down as much as possible to improve integration of the semiconductor device.
The chrome mask inherently has an error caused by light diffraction. That is, it is difficult to achieve a high accuracy of the circuit pattern with the chrome mask. Accordingly, the photo mask using the chrome light-shielding film has limitations in forming a fine-sized circuit pattern. Phase shift masks have been studied to solve this difficulty.
The phase shift mask is based on the diffraction and interference that occurs when transmitted light between adjacent patterns is mutually out of phase by 180°. Examples of the phase shift mask include a chrome-less phase shift mask, an attenuated phase shift mask, a phase-edge phase shift mask, and an alternating phase shift mask. Among them, the chrome-less phase shift mask allows a circuit pattern to be formed with phase shift patterns without the use of chrome.
The chrome-less phase shift mask comprises phase shift hillock patterns and phase shift pitting patterns. Both the phase shift hillock pattern and the phase shift pitting pattern transmit light. Light transmitted by the phase shift hillock pattern and the phase shift pitting pattern are mutually out of phase by 180°. Light sources having a mutual phase difference of 180° cause interference. With this interference, a predetermined pattern is formed on the photoresist. The given pattern formed by the interference may be adjusted using the phase shift hillock pattern and the phase shift pitting pattern. That is, an ideal arrangement of the phase shift hillock patterns and the phase shift pitting patterns allows the chrome-less phase shift mask to sophisticatedly form a finer pattern, compared to the chrome mask.
The chrome-less phase shift masks may be classified into a mesa scheme and a trench scheme, depending on their kinds of dark patterns. The mesa scheme implements the dark patterns by means of the phase shift hillock patterns while the trench scheme implements the dark patterns by means of the phase shift pitting patterns.
Semiconductor devices often include an area having a relatively wide circuit width and an area having a relatively narrow circuit width in a single chip. For example, the area having the wider circuit width may be a peripheral circuit area and the area having the narrower circuit width may be a cell area. In general, a light source used in an exposure device has a predetermined range of wavelengths. For this reason, a non-patternable region results depending on a dark-pattern implementing scheme. For example, in the mesa scheme, if the light source has a wavelength λ of 193 nm, a region having an after develop inspection critical dimension (ADI CD) of less than 80 nm can be patterned. To form both a pattern having an ADI CD greater than 80 nm and a pattern having an ADI CD of 80 nm, a region having an ADI CD of 80 nm should be formed in the mesa scheme while a conventional chrome light-shielding film should be formed on the remaining region. Similarly, in the trench scheme, a region having an ADI CD less than 90 nm can be patterned. To form both a pattern having an ADI CD greater than 90 nm and a pattern having an ADI CD of 90 nm, a region having the ADI CD of 90 nm should be formed in the mesa scheme and a conventional chrome light-shielding film should be formed on the remaining region.
This limits the use of the chrome-less phase shift mask, complicates the fabrication process, and degrades patterning accuracy in a non-patternable region.